1. Field of the Invention
The present invention relates to semiconductor laser structures and finds application in optical communications.
2. Description of Related Art
The radiation used in optical communications is not necessarily strictly in the visible region. If silica optical fibres are used as the transmission medium, infra-red radiation is of special usefulness since the loss minima occur in such fibres at 1.3 .mu.m and 1.55 .mu.m approximately. (The above wavelengths are in vacuo wavelengths as are all wavelengths herein except where otherwise specifically stated).
Semiconductor lasers are known which will emit optical radiation at wavelengths including the above. In general a semiconductor laser structure will include a p-n junction across which a driving current can be applied (the conventional current from p to n), an "active region" in which electrons and holes combine, producing photons by stimulated emission, and a feedback structure to cause oscillation of radiation in the active region. The active region has to relate suitably in band gap and refractive index to the other semiconductor regions of the structure in order to achieve a suitable degree of "confinement" of these processes to the active region. Lasers operating to produce optical radiation in the wavelength range from 1.1 to 1.65 .mu.m, especially from 1.3 to 1.6 .mu.m, usually comprise regions of indium phosphide and of the quaternary material (In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y). By suitable choices of x and y it is possible to lattice-match the various regions while varying the band gaps of the materials. (Band gaps can be determined experimentally by for instance photoluminescence). Additionally, both indium phosphide and the quaternary materials can be doped to p- or n- type as desired.
Semiconductor lasers comprising regions of gallium arsenide and gallium aluminium arsenide are also used for communications purposes. They operate near to 0.9 .mu.m.
The feedback structure might comprise for instance reflective end facets of the laser (Fabry-Perot or FP lasers). Alternatively it might comprise a grating of corrugations which lie near the active region (distributed feedback or DFB lasers). The corrugations may be shifted to a position external to the laser structure, adjacent the emission axis (a distributed Bragg reflector or DBR laser), or a reflective interface may be used in a cavity external to the laser.
Semiconductor lasers may vary otherwise in structure. For instance a double heterostructure (DH) laser may comprise an (initially undoped) active region of a first material, lying between a lower n-doped layer and an upper p-doped layer each of a different material. In a multiple quantum well (QW) laser, the active region comprises one or more very thin layers of active material, and where a plurality of such layers are provided, they are separated by barrier layers. The thin active layers are each so thin that that dimension is comparable to the De Broglie wavelength for electrons, and the discreteness of the density distribution of energy states in the active material becomes apparent. Quantum well layers have thicknesses (known as well width) typically of the order of 10 nm and the barrier layers may be still thinner.
Semiconductor laser structures are used in devices other than optical sources. For instance, if the driving current applied is less than a lasing threshold driving current, a laser structure may still be useful as an optical amplifier, amplifying an optical radiation input signal. Further, a laser structure may be used as an absorption modulator, a device coupled to a laser output port which can be switched between optically opaque and transparent conditions with respect to the laser output and so modulate it.
Laser structures vary considerably from each other in a number of operating characteristics. A known characteristic, which affects the modulation performance of directly modulated lasers, as well as laser emission linewidth, is the linewidth enhancement factor ".alpha.".
In directly modulated lasers, transient wavelength chirping is a phenomenon which occurs for instance either when a laser is switched on and off to produce a directly modulated output signal, or when an absorption modulator is used. It has the effect of distorting the oscillation frequency of an optical pulse, the pulse then tending to spread during transmission in a dispersive medium and causing difficulty in detection.
In some applications, particularly coherent optical communications, it is important that the continuous wave emitted radiation shows a narrow linewidth. This allows coherent detection systems, such as heterodyne or homodyne detection, to be used which are more sensitive than direct detection and give better bandwidth discrimination.
Unmodified FP lasers have been found unsuitable, having linewidths of more than 100 MHz. It is known that DBR and DFB lasers can be fabricated having narrower emission linewidths and that external structures such as external cavities can result in narrowed emission linewidths. However, both emission linewidth and transient wavelength chirping have tended to remain unpredictable characteristics in the design of laser structures.
Considerable work has been done in trying to assess linewidth in a laser structure. In the paper "Theory of the Linewidth of Semiconductor Lasers" by Charles H Henry, IEEE Journal of Quantum Electronics, OE-18(2), February 1982 pp 259-264, a theory is presented which arrives at a linewidth broadening term (1+.alpha..sup.2). Theoretical models for the gain and for .alpha., assuming homogeneous broadening, have been published for instance in papers by Y Arakawa and A Yariv, references IEEE J Quantum Electronics OE--21 starting on page 1666, 1985, and OE--22 starting on page 1887, 1986. Good agreement has been demonstrated with experimental data in GaAs quantum wells. However, the models have necessitated a large degree of numerical computation and the design of optimal structures has been complicated as a result.
In the paper "Dispersion of the Linewidth Enhancement Factor in Semiconductor Injection Lasers", by N. OGASAWARA et al, Japanese Journal of Applied Physics 23 1984 pp L518 to L520, it is predicted that the value of .alpha. decreases with increasing excitation frequency. However the relationship is complicated and difficult to apply to the design of laser structures for particular purposes. In addition, there is a practical problem in working at the high excitation frequencies, in that absorption also increases and the laser structure loses its transparency.